Coke drum with air injection system for skirt junction

ABSTRACT

A coke drum includes a coke drum vessel having a substantially cylindrical wall and a conical bottom portion; a skirt downwardly depending from a transition of the cylindrical wall into the conical bottom portion, an inner annular space being defined between the skirt and the conical bottom portion; and a fluid injection system communicated with the inner annular space for injecting fluid at a desired temperature to heat or cool the inner annular space. The fluid injection system allows the inner annular space and junction of the skirt with the coke drum vessel to be heated or cooled to minimize temperature difference between these areas and a batch of coke to be introduced into the drum.

BACKGROUND OF THE INVENTION

The invention relates to coke drums and more particularly to reducingstresses in the coke drum during heating and cooling of the coke drum.

Coke drums and delayed coke drums are vertical thin-walled pressurevessels that operate under severe conditions by cyclic heating andquenching operations. In use, a drum is filled with a hot coker feedand, after a period of time, the drum is emptied and prepared for thenext filling. During filling with hot coker feed, the drum is exposed tothe very high temperature of the hot coker feed, which can be as high as900° F. or more. When the drum is to be emptied, in one step a quenchwater can be introduced into the drum, and at this stage, the drum canbe much hotter than the quench water.

Because of these extreme temperature cycles, coke drums are one of thepressure vessels that register most failure frequency in refineries.According to an API survey, skirt cracking was reported by 73% ofcompanies surveyed who were using coke drums. In order to repair cracksin the skirt, operational windows are required, which requireinterruption of processes and, as a consequence, loss of opportunitiesand higher operational costs.

In addition, incidents have been reported associated with hydrocarbonleaks and incipient fires, creating certain operational safetyconditions. Studies have recognized that the cracks in coke drums resultfrom low cycle fatigue induced by cyclic thermal stress. In order toattempt to address this issue, coke drums have been provided with a zonecalled a “hot box” which closes off a space between the skirt and theconical bottom portion of the coke drum. This helps to increase theskirt temperature during a heating stage in order to decrease thethermal effect. This is not enough, however, to avoid problems withskirt cracking.

The need remains for a solution to the problem of skirt cracking.

SUMMARY OF THE INVENTION

According to the invention, a solution to skirt cracking is provided.Specifically, a fluid injection system is provided to inject fluid of anappropriate temperature into the “hot box” and thereby avoid the severethermal stresses normally encountered with coke drum operation.

According to the invention, a coke drum is provided which comprises acoke drum vessel comprising a substantially cylindrical wall and aconical bottom portion; a skirt downwardly depending from a transitionof the cylindrical wall into the conical bottom portion, an innerannular space being defined between the skirt and the conical bottomportion; and a fluid injection system communicated with the innerannular space for injecting fluid at a desired temperature to heat orcool the inner annular space.

The fluid injection system can be provided as a toroid tube in the innerannular space and communicated with a source of the fluid.

The fluid injection system can have a fluid inlet and a fluid outlet,and at least one baffle in the inner annular space between the fluidinlet and the fluid outlet.

The coke drum can have a bottom wall section connected between the skirtand the conical bottom portion and closing off the inner annular spaceto define the so-called “hot box”.

The fluid injection system can advantageously have a first toroid tubedefining a fluid inlet, a second toroid tube defining a fluid outlet,and a series of baffles in the inner annular space between the firsttoroid tube and the second toroid tube.

The series of baffles can be arranged so that they lead the fluid fromthe first or inlet toroid tube toward the second or outlet toroid tube,and positioned to provide that the fluid is always in contact with theskirt and conical surfaces of the drum. In one configuration, thebaffles can be positioned between the skirt and conical surfaces with agap between the baffle and these surfaces such that flow of fluid isguided along the surface of the skirt or conical section.

The fluid injection system can be provided so as to inject fluid aroundan entire circumference of the inner annular space.

A process for operating the system according to the invention, and alsoa method for retrofitting or adapting an existing coke drum to includethe present invention, are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of preferred embodiments of the inventionfollows, with reference to the attached drawings, wherein:

FIG. 1 is a schematic representation of a coke drum;

FIG. 2 shows an enlarged portion of FIG. 1 showing a “hot box” portionof the coke drum with a fluid injection system according to theinvention;

FIG. 3 shows a further enlarged portion of FIG. 2 including bafflesaccording to a further embodiment of the invention; and

FIG. 4 shows skirt and cone temperature over time for a coke drumwithout the fluid injection system of the present invention (solidlines) as compared to one with the fluid injection system of the presentinvention (dashed lines).

DETAILED DESCRIPTION

The invention relates to a coke drum having a fluid injection system forreducing thermal stress and increasing lifetime of a coke drum and,particularly, increasing lifetime of a junction between the cone andskirt of a coke drum.

FIG. 1 illustrates a coke drum 10 which typically has a substantiallyvertically oriented cylindrical portion 12 closed off at a top portion14 and having a substantially conical bottom portion 16. Further, askirt 18 is typically attached to coke drum 10 at a transition point 20from cylindrical portion 12 to conical bottom portion 16. Skirt 18 canbe attached to coke drum 10, most typically by welding.

Skirt 18 is typically used to support a coke drum during use. In use,coke drum 10 is filled with a hot coker feed and, after a period oftime, the drum is emptied and prepared for the next filling. At twodifferent stages in the use of a coke drum, the drum, including thejunction of skirt and transition point, are normally subjected toextreme changes in temperature. As will be described herein, thesechanges in temperature are moderated and controlled so as to reducethermal stresses through the drum and, particularly, at the junction ofskirt and cone, and thereby extend the life of the drum.

FIG. 2 is an enlarged view of the portion indicated in FIG. 1 at A, andshows a portion of a substantially cylindrical wall 22 which extendsaround axis X to define a coke drum vessel. This wall thereby defines asubstantially closed inner portion into which a coker feed is introducedfor cooling and subsequent treatment.

As shown, wall 22 transitions at the transition point 20 into aninwardly tapered wall 24 which defines conical bottom portion 16.

Skirt 18 is shown connected to the cylindrical drum and downwardlydepending from transition point 20. This connection of skirt 18 and wall22/24 is therefore a junction of these components. At this junction, thecomponents are typically welded, and it is the protection of this weldthat is the focus of the present invention.

It should be appreciated that skirt 18 and wall 24 define between theman enclosed inner annular space 26. A radial wall 28 can also beincluded, for example extending between skirt 18 and wall 24 to closeoff a portion of inner annular space 26. This portion is referred to asthe “hot box” 30.

FIG. 3 is an enlarged portion of FIG. 2 and shows tubes 32, 34 whichdefine fluid inlet and outlet, respectively, for use in introducing afluid to the hot box 30. Tubes 32, 34 can, for example, be provided astoroid tubes and are preferably disposed around a circumference ofconical bottom portion 16 in the hot box 30 or inner annular space 26defined between wall 24 and skirt 18. Tubes 32, 34 are communicated witha source of fluid at a variable temperature such that fluid can beintroduced through tubes 32, 34 to either heat or cool the hot box 30 asdesired.

FIG. 3 shows a further enlarged portion of FIG. 2, particularly theportion indicated at B. As shown, one tube 32 is configured as an inletand the other tube 34 is configured as an outlet for the fluid flow tobe introduced into hot box 30. Further, as shown in this illustration aseries of baffles 36 can advantageously be positioned in hot box 30between inlet tube 32 and outlet tube 34 such that fluid introducedthrough inlet tube 32 must travel a circuitous route to reach outlettube 34. This enhances the heat transfer from such fluid through thebaffles 36 and into skirt 18 and wall 24 as well as the weld betweenthese components. As shown, baffles 36 are preferably positioned toextend between the walls without reaching them, and this configurationserves to guide flow of fluid along the surfaces of the conical sectionand skirt as desired, for example as shown by the arrows in FIG. 3. Alsoas shown in FIG. 3, the toroid tubes 32, 34 and baffles 36 define afluid flow area which extends roughly the entire height h of hot box 30,and also which advantageously can extend around an entire circumferencec of the hot box area.

As set forth above, coke drums in general and the weld at the junctionof skirt 18 and wall 22, 24 are subjected to extreme changes intemperature in two cycles of typical use of a coke drum. The first iswhen a drum is filled with a hot coker feed, which can have atemperature as high as 900° F. or more. According to the invention,fluid can be injected into hot box 30 through tube 32 so as to heat thetemperature in hot box 30 and adjoining wall sections and weld to atemperature close to the expected temperature of incoming hot cokerfeed. In this way, when the hot coker feed is introduced into the drum,a AT between components of the drum and the hot coker feed can beminimized. Still further, ΔT between the skirt and the cone can beminimized as well, and this most advantageously reduces thermal stressesin the weld connection at the junction between these components.

Thus, in advance of filling a coke drum with hot coker feed, theexpected temperature of the hot coker feed can be determined usingvarious means well known to the person of ordinary skill in the art, andhot fluid can then be introduced through tube 32 into hot box 30 at atemperature sufficient to elevate temperature of wall 24 and skirt 18 tosubstantially the same temperature as the expected incoming hot cokerfeed. Once the hot coker feed has rested in the coke drum and it is tobe removed, a quench step is typically conducted where quench water isintroduced into the drum. In advance of this step, fluid can beintroduced through inlet tube 32 which is at a temperature suitable toreduce temperature of the hot box 30 and walls defining same to besubstantially the same as the temperature of the expected quench water.In this way, during quenching, AT between skirt and cone can also beminimized.

The fluid to be used in the present invention can be any type of fluidwhich would be suitable for delivering the desired amount of heatingand/or cooling to hot box 30. According to one embodiment of theinvention, this fluid is advantageously air.

In the typical environment of use of the apparatus of the presentinvention, air is readily available from a number of different sourcesand at a number of different temperatures, and this thereforeadvantageously facilitates deployment of systems in accordance with thepresent invention. Of course, if desirable for other reasons differentfluids could be used.

It should also be appreciated that all components defining the hot box,including tubes 32, 34 and baffles 36, should be fabricated frommaterials which are well resistant to any detrimental effects of beingexposed to hot air.

In order to demonstrate the advantageous results of use of the presentinvention, tests were taken using a coker drum without a fluid injectionsystem according to the present invention, and a coker drum with thefluid injection system of the present invention.

In the course of these tests, temperature of the skirt section and thecone section were monitored during a full cycle of use of a coke drum.Results are shown in FIG. 4. In FIG. 4 a solid line referred to atreference numeral 38 shows cone temperature over time during the test,for a coke drum without the fluid injection system of the presentinvention. As shown, from a starting point of approximately 65° C., thetemperature of the cone upon filling rapidly climbed, over the period ofapproximately 6.5 hours to a temperature of approximately 390° C. Stillreferring to FIG. 4, the solid line indicated by reference numeral 40 isthe corresponding skirt temperature over time, and FIG. 4 shows thatonce the drum is filled, there is a ΔT between cone 38 and skirt 40 of30° C. or more. Further, these large ΔT values are throughout theprocess, including times of steep temperature changes, for exampleduring introduction of a hot coker feed, or at the time of quenching.

Still referring to FIG. 4, the dashed line indicated at 42 shows conetemperature for a coke drum including a fluid injection system inaccordance with the present invention. Further, the skirt temperature ofthis device is shown in FIG. 4 at reference numeral 44. As shown, thespacing between dashed lines 42, 44 or ΔT when the fluid injectionsystem of the present invention is used, is significantly smaller thanthe ΔT in the drum without a fluid injection system. This advantageouslyreduces thermal stresses to which the junction between the skirt andcone is subjected, and thereby increases the useful life of the cokedrum and, particularly, the welds at this junction.

It should therefore be appreciated that the system in accordance withthe present invention is advantageously used in a method whereintemperature of a coker feed to be delivered to the drum is determined,fluid of a desired temperature is then introduced through the fluidinjection system and inlet tube 32, to bring the temperature of hot box30 including both cone and skirt, to a substantially similar temperaturein advance of introduction of the coker feed into the drum.

Once the coker feed in the drum has rested a sufficient period of time,and the drum is to be emptied and cleaned, the temperature of aquenching water can be determined and additional fluid injected to thehot box to cool the hot box to substantially the same temperature as thequenching water. In this way, the ΔT during quenching can also beminimized in accordance with the present invention.

It should also readily be appreciated that the fluid injection system ofthe present invention can be easily adapted to incorporate into anexisting coke drum without such a fluid injection system. This could bedone, for example, by introducing tubes 32, 34 into the inner annularspace 26 defined by skirt 18 and wall 24 and further by adding baffles36 and a radial wall 28 if necessary, such that an existing coke drumcould then be protected with the system of the present invention.

It is to be understood that the invention is not limited to theillustrations described and shown herein, which are deemed to be merelyillustrative of the best modes of carrying out the invention, and whichare susceptible of modification of form, size, arrangement of parts anddetails of operation. The invention rather is intended to encompass allsuch modifications which are within its spirit and scope as defined bythe claims.

We claim:
 1. A coke drum, comprising: a coke drum vessel comprising asubstantially cylindrical wall and a conical bottom portion; a skirtdownwardly depending from a transition of the cylindrical wall into theconical bottom portion, an inner annular space being defined between theskirt and the conical bottom portion; and a fluid injection systemcommunicated with the inner annular space for injecting fluid at adesired temperature to heat or cool the inner annular space.
 2. The cokedrum of claim 1, wherein the fluid injection system comprises a toroidtube in the inner annular space and communicated with a source of thefluid.
 3. The coke drum of claim 1, wherein the fluid injection systemhas a fluid inlet and a fluid outlet, and at least one baffle in theinner annular space between the fluid inlet and the fluid outlet.
 4. Thecoke drum of claim 1, further comprising a bottom wall section connectedbetween the skirt and the conical bottom portion and closing off theinner annular space.
 5. The coke drum of claim 1, wherein the fluidinjection system comprises a first toroid tube defining a fluid inlet, asecond toroid tube defining a fluid outlet, and a series of baffles inthe inner annular space between the first toroid tube and the secondtoroid tube.
 6. The coke drum of claim 5, wherein the series of bafflesdirect flow of fluid along the skirt and conical surfaces of the drum.7. The coke drum of claim 1, wherein the fluid injection system injectsfluid around an entire circumference of the inner annular space.
 8. Aprocess for protecting a coke drum from thermal stresses, comprising thesteps of: determining coke temperature of coke to be introduced to acoke drum, wherein the coke drum comprises: a coke drum vesselcomprising a substantially cylindrical wall and a conical bottomportion; a skirt downwardly depending from a transition of thecylindrical wall into the conical bottom portion, an inner annular spacebeing defined between the skirt and the conical bottom portion; and afluid injection system communicated with the inner annular space forinjecting fluid at a desired temperature to heat or cool the innerannular space; and introducing fluid at a temperature substantiallysimilar to the coke temperature through the fluid injection system andinto the inner annular space, whereby temperature of the inner annularspace and a junction of the skirt with the coke drum vessel is adjustedto be substantially similar to the coke temperature.
 9. The process ofclaim 8, wherein the fluid injection system comprises a toroid tube inthe inner annular space and communicated with a source of the fluid. 10.The process of claim 8, wherein the fluid injection system has a fluidinlet and a fluid outlet, and at least one baffle in the inner annularspace between the fluid inlet and the fluid outlet.
 11. The process ofclaim 8, further comprising a bottom wall section connected between theskirt and the conical bottom portion and closing off the inner annularspace.
 12. The process of claim 8, wherein the fluid injection systemcomprises a first toroid tube defining a fluid inlet, a second toroidtube defining a fluid outlet, and a series of baffles in the innerannular space between the first toroid tube and the second toroid tube.13. The process of claim 12, wherein the series of baffles direct flowof fluid along the skirt and conical surfaces of the drum.
 14. Theprocess of claim 8, wherein the fluid injection system injects fluidaround an entire circumference of the inner annular space.
 15. A methodfor adapting an existing coke drum, comprising the steps of: providingan existing coke drum having a substantially cylindrical walltransitioning to a conical bottom portion, and having a skirt downwardlydepending from a transition from the substantially cylindrical wall tothe conical bottom portion; and positioning a fluid injection systemrelative to an inner annular space defined between the skirt and conicalbottom portion whereby temperature of a junction of the skirt and thecoke drum can be adjusted by introducing fluid at a desired temperaturethrough the fluid injection system and into the inner annular space.